In semiconductor manufacturing, ion implantation is used to change material properties of portions of a substrate. Indeed, ion implantation has become a standard technique for introducing conductivity-altering impurities into semiconductor wafers to produce various semiconductor-based products. In an ion implantation process, a desired impurity material is typically ionized in an ion source, ions are accelerated to form an ion beam of prescribed energy, and the ion beam is directed at a surface of a semiconductor wafer. Energetic ions in the ion beam penetrate into bulk semiconductor material and are embedded into a crystalline lattice of the bulk semiconductor material of the wafer to form a region of desired conductivity.
Ion implantation systems typically include an ion source for converting a gas or a solid material into a well-defined ion beam. The ion beam may be mass analyzed to eliminate undesired species, accelerated to a desired energy, and then directed onto a target area, typically a surface of a wafer of semiconductor material. The ion beam may be distributed over the target area by beam scanning, target area movement, or by a combination of beam scanning and target area movement. Examples of conventional ion implanters are disclosed in U.S. Pat. No. 4,276,477 issued Jun. 30, 1981 to Enge; U.S. Pat. No. 4,283,631 issued Aug. 11, 1981 to Turner; U.S. Pat. No. 4,899,059 issued Feb. 6, 1990 to Freytsis et al.; U.S. Pat. No. 4,922,106 issued May 1, 1990 to Berrian et al.; and U.S. Pat. No. 5,350,926 issued Sep. 27, 1994 to White et al.
Ion species presently used for ion implantation in silicon include arsenic, phosphorus, germanium, boron, and hydrogen having energies that range from below 1 keV to above 80 keV. Ion currents ranging from microamperes to multi-milli-amperes are typically needed. Tools providing implant currents greater than about 0.5 mA are commonly referred to as “high-current” implanters. Trends within the semiconductor industry are moving towards ion implantation energies below 1 kev and control of angle of incidence below 1 degree.
A recent improvement for ion implantation systems has been the introduction of ribbon beam technology. In such a technology, ions arriving at a work piece are organized into a ribbon ion beam that coats the work piece uniformly as the work piece is passed under the beam. There are significant cost advantages of using such ribbon beam technology. For example, for disc-type implanters, ribbon-beam technology eliminates the necessity for scanning a disc across an ion beam. For single-wafer implanters, a wafer need only be moved under an incoming ribbon beam along a single dimension, greatly simplifying the mechanical design of end-stations and eliminating a need for transverse electromagnetic scanning. Using a properly shaped ribbon beam, uniform dosing density is possible across a work piece with a single one-dimensional pass.
Technical challenges of generating and handling ribbon beams are non-trivial because ribbon beam/end station arrangements typically must produce dose uniformities better than 1%, angular accuracies better than 1 degree, and operate with ion energies below 1 keV. U.S. Pat. No. 5,350,926 entitled “High current ribbon beam ion implanter” and U.S. Pat. No. 5,834,786, entitled “Compact high current broad beam ion implanter”, both issued to White et al., present some features of ribbon beam technology.
In a ribbon beam ion implanter, although relatively low energy may be used at a workpiece, it is often desirable to transport an ion beam over most beam line elements at a relatively higher energy. A reason for this is that transporting an ion beam at lower energy levels required at a final stage would cause space charge blow-up to occur. Thus, in some ion implanting devices a decelerator is used to reduce ion energy to a desired level just prior to ion beam incidence upon a substrate.
A common problem with conventional ribbon beam implanters is that some ions may lose their charge in a charge exchange reaction with residual gas along a beam line. Ions that have incurred a charge exchange reaction (referred to herein as “neutrals”) are unaffected by a decelerator. Thus, they are likely to impact a target substrate at a higher energy level. This high energy contamination of the target substrate is undesirable and obviously may have a negative impact on a resulting implanted device.
One method of mitigating affects of neutrals is to apply an energy filter just after a decelerator stage in an ion implanter. Such an energy filter is usually effective at preventing high energy neutrals from reaching a target. However, a shortcoming of this solution is that a decelerated, low energy beam is very difficult to transport even over small distances because it is subject to large space charge blow up. Thus, transporting the beam through an energy filter becomes problematic because it will tend to be overly inclusive. That is, it will undesirably attenuate desired ions and prevent them from reaching a substrate with a desired energy and at a desired concentration. Also, only a limited amount of current may be transported through such filter, often with significant degradation of beam parallelism.
In view of the foregoing, it would be desirable to provide a technique for separating ions from neutrals in a ribbon beam ion implanter that overcomes some or all of the above-described inadequacies and shortcomings of known systems.